Cannabis plants with an estimated street value of £3m have been seized from a former bingo hall.
South Wales Police officers found about 3,000 cannabis plants growing in 20 different rooms of the building on Broad Street in Barry, Vale of Glamorgan.
They were called on 13 March after National Grid officials raised concerns about energy supply.
No arrests have been made and police said the investigation was ongoing.
There was nobody in the building when officers arrived, but they found kitchen facilities, food and clothing suggesting that people had been staying there.
They also found thousands of pounds worth of equipment, including lighting, fans and fertiliser.
“Many people think it is ‘just a bit of cannabis’ but any production of drugs is linked to hidden harm,” said PC Beth Harrison.